Permeable, porous polymeric membrane with hydrophilic character, methods for preparing said membranes and their use

ABSTRACT

Permeable, porous polymeric membrane with hydrophilic character of the membrane, said character being obtained by treatment with a solution comprising one or more hydrophilic, mono-or polymeric compounds selected among soluble, OH-containing cellulose derivatives, polyvinylalcohols and low molecular weight, polyfunctional, NH-and/or OH-containing compounds, optionally in the presence of a cross-linking agent, a surfactant and a initiator followed by rendering the layer deposited during the treatment insoluble on the membrane surface by means of a catalyst reaction at elevated temperatures in order to fixate the hydrophilic material to the membrane. Furthermore a process for the preparation of these membranes by means of treating a permeable, porous, polymeric membrane of hydrophobic character with the above solution in the presence of a catalyst and optionally in the presence of a cross-linking agent, a surfactant and an initiator, and curing of the coated membrane by means of heating to form an insoluble, hydrophilic surface layer. The membranes are suitable for use in ultra- and microfiltration.

This application is a continuation of application Ser. No. 07/089,550,filed on Aug. 26, 1987, now abandoned.

FIELD OF THE INVENTION

The invention relates to permeable, porous polymeric membranes withhydrophilic character, methods for preparing said membranes and theiruse for ultra- and microfiltration.

BACKGROUND ART

Ultra- and microfiltration membranes (UF- and MF membranes) are todaywidely used in a large number of industries for separation andconcentration of solutions and suspensions.

The word membrane is in this context used to describe thin, porousfilms, foils and sheets, usually in form of permeable flat films, tubesor hollow fibers prepared from various organic types of polymers. UFmembranes are characterized by their ability to separate macromoleculesfrom low molecular weight compounds and salts. Membranes are specifiedby the molecular weight cutoff value, (MWCO). The cutoff value normallyrefers to well-defined macromolecules (globular proteins) and indicatesthat macromolecules with a molecular weight higher than the given MWCOare not allowed to pass the membrane, while molecules with a lowermolecular weight more or less pass the membrane. MF membranes arecharacterized by their ability to retain bacteria and othermicroorganisms, particles, colloids etc. and are specified by theindication of their pore size e.g. determined by the so-called BubblePoint Method as stated in ASTM F316 (Standard Method of Test for PoreSize Characteristics of Membrane for use with Aerospace Fluids).

The expression "hydrophobic membrane" refers in the following to anymembrane made more hydrophilic by the method according to the invention.

UF and MF membranes are used in a large number of industries, such aswater processing, food, chemistry, pharmaceutics, bio-engineering, wastewater processing etc., for various separation and concentration tasks,removal of particles and the like.

In order to be usable the membranes have to meet several requirements,such as

Sufficient physical strength for resisting the mechanical influences themembranes are subjected to during operation in the filtration system inquestion.

Good chemical resistance, allowing use of the membranes in a widetemperature and pH range in different chemical environments as well astoleration of various detergents and desinfectants.

Good transport properties (flux) and selectivity allowing a givenseparation/concentration task to be technically and economicallyfeasible.

UF and MF membranes are usually made of highly resistant, syntheticpolymers more or less hydrophobic by nature and displaying a greattendency for ad- and absorption of many organic compounds and colloids.

Ad- and absorption results in lower flux values as well as a fluxreduction with time (fouling). At the same time the separationproperties may be altered, since the ad- and absorbed material on thesurface of the membranes forms a layer, said layer being able to act asa secondary membrane.

A number of the above disadvantages are avoided if hydrophilic insteadof hydrophobic membranes are used, said hydrophilic membranes beingeasily wetted by water and known to possess low ad- and absorption offouling materials (e.g. proteins). They generally possess anadvantageously low protein binding tendency and a lesser inclination forfouling, the latter in turn resulting in advantages in form of higherflux values, lesser flux reduction with time as well as easier cleaningand lower cleaning frequency.

Furthermore, it is thus possible to obtain membranes with a lower cutoffvalue and the membranes may be stored in dry condition, since they areeasily rewetted with water.

The British patent specification 1.538.810 describes thehydrophilization of fluoropolymers with water-soluble polymers (i.e.cellulose derivatives and polyvinylalcohol) by rendering said polymersinsoluble by means of cross-linking reactions, such as heat treatment,acetalization, esterification, chemical reaction with bichromate, or bymeans of ionizing radiation. In the present invention, however, thewater-soluble, OH-containing polymers are rendered insoluble either byformation of ether bonds between the polymer chains by means of lowmolecular weight cross-linking agents or by formation of chemical bondsbetween said polymers and a polyvinylidenefluoride and/orpolyvinylidenefluoride copolymer support membrane by means of abase-catalyzed reaction. The above British patent specification does notrelate to the use of the hydrophilic, porous fluorocarbon structures,while the present invention improves the properties of the membrane.

The U.S. Pat. No. 3,620,970 describes the preparation of Reverse Osmosis(RO) membranes, i.e. membranes with salt retention properties. Theultrafiltration and microfiltration membranes of the present inventionpossess a insignificant salt retention (ideally 0% is desired) but animproved membrane surface in order to avoid or reduce ad- andabsorption/fouling during hydrophilization.

U.S. Pat. No. 4,340,482 discloses a microporous, polymeric articleattaining hydrophilic properties by means of treatment with a highlyalkaline solution of an amino acid, preferably glycine. Thehydrophilicity is attained by means of grafting with the amino acidbeing a charged molecule. Membranes with charged groups displaypH-related flux and permeability properties during the filtration ofmany products. Components with opposite charge may thus be bound to themembrane and foul said membrane, while components with the same chargeare repulsed and optionally completely or partially retained during thefiltration. The use of highly alkaline concentrations and hightemperatures in this known method limits its application.Hydroxypropylcellulose being used i.e. in the present invention cannotbe dissolved under these conditions.

The synthetic polymers usually used for the preparation of a membraneare distinctly hydrophobic, e.g. polysulfone. Natural polymers, such ascellulose and derivatives thereof, are, on the other hand, hydrophilicand today UF membranes are made of regenerated cellulose. The use of MFmembranes made of regenerated cellulose has, however, been problematicbecause of the ability of this material to swell in water as well as itslow mechanical strength due to its high porosity.

Further synthetic polymers include fluoropolymers, homopolymers as wellas vinylidene fluorides, such as polyvinylidene fluoride (PVDF)generally less sensitive to fouling than polysulfone. Ultrafiltrationmembranes made of PVDF have been known for many years and correspondingmicrofiltration membranes are also produced. One technique includesadding small amounts of a partially mixable fluoro-copolymer(chlorotrifluoroethylene/vinylidene fluoride copolymer (CTFE/VF)) to themembrane solution containing besides PVDF a common solvent, optionallyadmixed with swelling agents/non-solvents and viscosity-increasingcomponents. The preparation is performed with the traditional machineryin a manner known per se. Another technique makes use of thefluoro-copolymers (CTFE/VF) as sole membrane polymer, by using a highmolecular weight, water-soluble polymer as pore former.

In view of the good properties of cellulose it was natural toinvestigate the possibility of employing the so-called thin filmtechnique for coating polysulfone as well as the above PVDF-based MFmembranes. Cellulose is not directly suitable because of its dissolvingproperties. Thus it is necessary to use derivatives thereof.Hydroxyethylcellulose and hydroxypropylcellulose have been proven to besuitable, since they are water soluble. In order to fixate thesematerials on the membrane surfaces they have to be made water-insoluble.This is done either by means of cross-linking the polymer chains or bycreating a chemical bond to the support membrane. Polysulfone does notimmediately present any reaction possibilities, while PVDF as well asthe CTFE/VF copolymer present reaction possibilities under highlyalkaline conditions and elevated temperatures. Under these conditionshydrogen fluoride/hydrogen chloride are set free while reactive groupsand double bonds, the basis of an addition reaction, are formed.

According to the present invention UF and MF membranes with hydrophiliccharacter may be provided on the basis of existing ultrafiltration andmicrofiltration membranes. The process includes applying an aqueoussolution comprising a hydrophilic polymer and possible, reactivemonomers as well as a catalyst to the "surface". Then the membrane isheated, e.g. in an oven, to a suitable temperature so that a hydrophilic"surface layer" is formed, said layer being either chemically orphysically bound to the membrane material. Thus the properties of themembrane are altered in such a way that the character of the "surfacelayer" predominantly determines the filtration properties.

In the present specification the term "surface" in connection withmembranes refers to any membrane surface being able to come in contactwith the liquid to be filtered and thus not only to the outer surface ofsaid membrane.

SUMMARY OF THE INVENTION

It is thus the object of this invention to provide novel and improvedmembranes with hydrophilic character, said membranes being easily wettedand possessing good separation and non-fouling properties.

Another object of the invention is to provide a method for thehydrophilization of existing membrane types based on highly resistant,synthetic hydrophobic polymers.

A further object of the invention is to show special fields ofapplication for membranes according to the invention.

The objects of the invention are achieved by the membranes describedhereinafter, the methods described hereinafter, and the applicationdescribed hereinafter.

The present invention relates to a permeable, porous polymeric membranewith hydrophilic character wherein the hydrophilic character of themembrane is obtained by treatment with a solution comprising one or morehydrophilic, mono- or polymeric compounds selected among soluble,OH-containing cellulose derivatives, polyvinylalcohols and low molecularweight, polyfunctional, NH- and/or OH-containing compounds, optionallyin the presence of a cross-linking agent, a surfactant and an initiatorfollowed by rendering the layer deposited on the membrane surfaceinsoluble on the membrane surface by means of a catalyzed reaction atelevated temperatures in order to fixate the hydrophilic material to themembrane.

The hydrophilic compounds are, as described, applied in solution. Anysolvent not attacking the support membrane may be used and thereforewater is usually used since it does not influence the membrane.

The theory behind the obtained results is not explained in detail and itis not desired to be bound by a specific theory, but apparently thereexists a chemical bond between the hydrophilic compound and themembrane, or a physical bond (absorption) to the membrane or anintermolecular relation is obtained by the interreaction (cross-linking)between the polymer chains themselves and/or by cross-linking by meansof cross-linking agents.

The invention relates more specifically to a membrane wherein itshydrophilic character is obtained by treatment with a solutioncomprising one or more soluble cellulose derivatives selected amonghydroxyalkylcellulose and other cellulose ethers not completelysubstituted, preferably among hydroxyethylcellulose,hydroxypropylcellulose, methylcellulose, ethylcellulose,carboxymethylcellulose and methylhydroxyethylcellulose and/orpolyvinylalcohol. Suitable cellulose derivatives are thus derivativessoluble in a wide pH range and comprising free OH-groups, which mayenter into the final curing process. The cellulose polymers used asmembrane coating material are furthermore generally characterized bypossessing a low ad- and absorption tendency towards fouling materials,such as proteins, in such a way that such membranes according to theinvention possess non-fouling properties. Among the above advantageouscompounds hydroxyethyl- and hydroxypropylcellulose are at the momentpreferred, since they achieve the greatest effect.

According to another, preferred embodiment of the invention the lowmolecular weight, polyfunctional compounds are selected among primaryand secondary amines, compounds of the general formula ##STR1## whereR=--NHR², --OH, --SO₃ H or --COOH, and R¹ and R² are independentlyeither H or an alkyl group and diaminopyridines. Especially preferredlow molecular weight compounds are diaminopyridines, tetraethylenepentamine, aminophenols and sulphanilic acid.

According to a further preferred embodiment of the invention thecellulose derivative is completely or partially substituted by one ormore low molecular weight, polyfunctional compounds selected amongprimary and secondary amines, compounds of the general formula ##STR2##where R=--NHR², --OH, --SO₃ H or --COOH, and R¹ and R² are independentlyeither H or an alkyl group, and diaminopyridines.

The invention further relates to a process for preparing permeable,porous polymeric membranes, said process being wherein a permeable,porous, polymeric membrane with hydrophobic character is treated with asolution comprising one or more hydrophilic, monomeric or polymericcompounds selected among soluble, OH-containing cellulose derivativespolyvinylalcohols as well as low molecular weight, polyfunctional, NH-and/or OH-containing compounds in the presence of a catalyst andoptionally in the presence of a cross-linking agent, a surfactant, andan initiator, said coated membrane being subsequently cured by means ofheating until an insoluble, hydrophilic surface coat is formed. In theprocess according to the invention the hydrophilicity is obtained bymeans of treatment with preferably high molecular weight compounds. Thusit is possible to simultaneously influence the cutoff properties of theUF membranes, i.e. to prepare "tighter" membranes than those known inconnection with hydrophobic membrane material in question. The method isunusually simple and suitable for treating flat membranes. The chosensolutions may be applied to the surface of the membrane by means of e.g.a spreading technique, using a falling film of liquid or a spray. Inthose cases where it is important that also the inner membrane surfaces(pores) should be modified in connection with the filtration of foulingmaterials not retained by the outer surface of the membrane, a dippingtechnique is advantageously used for the application. For those purposesit is preferred to use the low molecular weight compounds, especiallyfor filtration purposes, where membranes with small pore size areinvolved.

In a preferred embodiment of the process according to the invention thecuring is performed by heating the membrane in a ventilated oven, saidmembrane advantageously passing through the oven at a suitabletemperature, e.g. between 100° C. and 175° C., for a suitable period,e.g. from 3 to 60 min. Other ways of heating may of course also be used,such as the use of radiation heat.

In a preferred embodiment of the process according to the invention thetreatment is performed in the presence of a common inorganic base,preferably sodium hydroxide, or a common inorganic acid, preferablyconcentrated sulphuric acid, as a catalyst. In this connection it shouldbe mentioned that the choice of catalyst is adapted to the solubility ofthe chosen coating material in alkaline or acidic medium respectively.Furthermore it should be mentioned that if the reactant is a highlyalkaline amine it may at the same time act as a catalyst for thereaction.

In a preferred embodiment of the process according to the invention amembrane is treated with a solution comprising hydroxyalkylcellulose inthe presence of an inorganic base as a catalyst, such as NaOH, possiblea cross-linking agent, such as a compound with the general formula R³-(CHOH--CH₂ --X)_(n), where X is a halogen atom, preferably chlorine, R³is an alkyl group that may include one or more oxygen or halogen atomsor a direct bond or an ether bond, and n is an integer equal to orlarger than 1, such as 1,3-dichloro-2-propanol, as well as optionally asurfactant, such as sodiumdodecyl sulphate.

The optionally used cross-linking agent is, as shown, a difunctionalcompound being able to react with hydroxyalkylcellulose and to formcross links between the polymer chains. 1,3-dichloro-2-propanol, forexample, forms ether bonds with hydroxyalkylcellulose via the OH groups.Other suitable cross-linking agents includeethyleneglycoldichlorohydrine, glyceroldichlorohydrine,glyceroltrichlorohydrine, sorbitoldichlorohydrine,sorbitoltrichlorohydrine and sorbitoltetrachlorohydrine.

Surfactants are used to facilitate the wetting of the hydrophobicmembrane with the coating material solution. Suitable surfactantsinclude compounds with no negative influence on the desired membrane,such as sodiumdodecyl sulphate (SDS).

In another preferred embodiment of the process according to theinvention a PVDF-, CTFE/VF- or a PVDF-CTFE/VF-membrane is treated with asolution comprising hydroxyalkyl cellulose in the presence of aninorganic base as catalyst, such as NaOH, optionally a cross-linkingagent, such as a compound with the general formula R³ -(CHOH--CH₂--X)_(n), where X is a halogen atom, preferably chlorine, R³ is an alkylgroup that may include one or more oxygen or halogen atoms or a directbond or an ether bond, and n is an integer equal to or larger than 1,such as 1,3-dichloro-2-propanol, as well as optionally a surfactant,such as sodiumdodecyl sulphate.

In such a base-catalyzed reaction between the coating material and themembrane material, PVDF, CTFE/VF and PVDF-CTFE/VF set free HF/HCl andform reactive centres for chemical bonds (grafting) under basicconditions and at high temperatures. The reaction mechanism forhydroxyalkylcellulose is, as mentioned before, not explained, but theresult of the treatment is that hydroxyalkylcellulose is bound to themembrane material.

In yet another preferred embodiment of the process according to theinvention the cellulose derivative is completely or partiallysubstituted by one or more low molecular weight, polyfunctional, NH-and/or OH-containing compounds, preferably selected amongdiaminopyridines, tetraethylene pentamine, aminophenols, and sulfanilicacid. In such cases a basic catalyst, such as NaOH is used.

In a further preferred embodiment of the process according to theinvention a membrane is treated with a solution comprising ahydroxyalkylcellulose and/or a polyvinylalcohol in the presence of aninorganic acid, preferably sulphuric acid, as catalyst, optionally across-linking agent selected among polyols, such as sugars, e.g.gluconic acid or sorbitol, inositol, pentaerythrite, aromatichydroxycompounds, e.g. 2,5-dihydroxybenzene sulfonic acid or 4,5dihydroxy-b 1,3-benzenedisulfonic acid, polyoxyethylene sorbitaneesters, polyfunctional acids, such as polyacrylic acid, polyfunctionalesters, such as dimethyltartrate and diethyltartrate, andpolyfunctional, unsaturated compounds, such astriallyl-1,3,5-triazine-2,4,6-trione, as well as a peroxide compound asinitiator if required. Suitable peroxide compounds include alkalinemetal peroxodisulphates, such as potassium peroxodisulphate

The reaction mechanism of the above acid-catalyzed reaction is notexplained either. It is, however, assumed, that the OH groups of thehydroxyalkylcellulose and the polyvinylalcohols form ester or etherbonds with subsequent cross-linking of the polymer chains under acidicconditions, at high temperatures and in the presence of a cross-linkingagent.

In another, further preferred embodiment of the process according to theinvention a polysulfone, a PVDF-, CTFE/VF- or a PVDF-CTFE/VF-membrane istreated with a solution comprising a hydroxyalkylcellulose and/or apolyvinylalcohol in the presence of an inorganic acid, preferablysulphuric acid, as catalyst, optionally a cross-linking agent selectedamong polyols, such as sugars, e.g. gluconic acid or sorbitol, inositol,pentaerythrite, aromatic hydroxycompounds, e.g. 2,5-dihydroxybenzenesulfonic acid or 4,5 dihydroxy-1,3-benzenedisulfonic acid,polyoxyethylene sorbitane esters, polyfunctional acids, such aspolyacrylic acid, polyfunctional esters, such as dimethyltartrate anddiethyltartrate, and polyfunctional, unsaturated compounds, such astriallyl 1,3,5-triazine-2,4,6-trione, as well as a peroxide compound asinitiator if required.

The hydrophilic character of the membranes prepared according to theinvention is determined by measuring the ability of said membranes to bewetted with water. The measure for the ability to be wetted is thecontact angle air water-membrane determined by the measuring methodaccording to SCAN p. 18-66 (Scandinavian Pulp, Paper and Board TestingCommittee), since it holds that the smaller the contact angle the betterthe ability to be wetted.

Furthermore the invention relates to the application of hydrophilic,permeable, porous, polymeric membranes described for ultrafiltration andmicrofiltration.

The principle of surface treatment can be shortly summarized as followswith the hydrophilic compound being represented by hydroxyalkylcelluloseand the catalytic reaction being basic and acidic respectively.

An aqueous solution of hydroxyalkylcellulose is prepared, and renderedalkaline by adding sodium hydroxide. A microfiltration membrane iscoated with the solution by dipping or pouring and excess solution isremoved by draining or squeezing. The membrane treated this way is thentransfered to a ventilated oven with a temperature of between 120°-150°C. for a suitable period, e.g. 10-15 min. The effect of this treatmentis evident from measuring the altered ability of the membrane to bewetted (the contact angle is decreased, as mentioned a sign for thehydrophilization of the surface) as well as from measuring the increasedflux values when filtering e.g. skimmed milk. Thus a fixation of thehydroxyalkylcellulose onto the membrane surface is proven. Thehydroxyalkylcellulose in the aqueous solution may completely orpartially be substituted by various low molecular weight, polyfunctionalNH- and/or OH-containing compounds. In order to achieve a furtherfixation of the hydroxyalkylcellulose a cross-linking agent, such as1,3-dichloro-2-propanol, may be added to the reaction solution.

If an acid-catalyzed reaction is chosen, a cross-linking agent as wellas an acid are added to the aqueous solution of hydroxyalkylcellulose.This mixture is then treated as described above, i.e. the membrane iscoated and heat-treated.

The basic membrane is preferably prepared of polyvinylidene fluoride,chlorotrifluoroethylene/vinylidene fluoride copolymer, polysulfones ormixtures of polyvinylidene fluoride andchlorotrifluoroethylene/vinylidene fluoride copolymer.

After the application the water-soluble cellulose derivative is renderedinsoluble and fixated onto the membrane material by means of heating,e.g. oven treatment.

DESCRIPTION OF THE PREFERRED EMBODIMENT Description of the preparationof known permeable membranes

Polymeric membranes are usually prepared from a solution of thecorresponding polymeric material in suitable solvents. The solution isshaped to form a thin film, a tube or a hollow fiber whereupon thepolymeric material is precipitated under controlled conditions. Theprecipitation may e.g. be performed by means of vaporization of thesolvent or by means of contacting the polymeric solution with anon-solvent. U.S. Pat. No. 3,615,024 describes such a method for thepreparation of porous, permeable, polymeric membranes. In the book"Reverse Osmosis/Ultrafiltration Process Principles" by S. Sourirajanand Takeshi Matsuura, National Research Council, Canada, 1985, NRCC no.24188 on p. 788-792 there is a description of the technique forpreparing flat membranes. An article by Douglas R. Lloyd and Timothy B.Meluch in "Material Science of Synthetic Membranes", D. R. Lloyd,editor, ACS-symposium Series 269, American Chemical Society, WashingtonD.C., 1985, p. 1-21 mentions examples of a number of different polymericmaterials usable for the preparation of membranes.

The membranes are often directly produced on a support surface thusconstituting an integrated part of the final membrane and providing goodmechanical strength. The UF and MF types used in the present inventionare thus reinforced with a support surface of non-woven polypropylene.

Preparation of microfiltration membranes based on a mixture ofPVDF-homopolymers and CTFE/VF-copolymers (the types used in theexamples):

The preparation method is based on the same principles as the abovemethod (phase inversion membranes), whereas the line of procedureenables a "control" of the pore size of the membranes (bubble point).

The basic polymer used is PVDF dissolved in a suitable concentration ina suitable solvent or a suitable solvent mixture. To this mixture isthen added

a) a fluoropolymer (PVDF-CTFE/VF-copolymer) soluble in the chosensolvent for PVDF and partially mixable with PVDF in said solvent,

b) optionally a water-soluble, high molecular weight polymer(polyvinylpyrrolidone) soluble in the above solvent and mixable with thePVDF-CTFE/VF-mixture.

Principle

The addition of additive a is to a great extent decisive for the poresize of the final membrane. An increase in concentration causes anincrease in pore size, as shown by measuring the bubble point. Theamount of additive a may vary greatly depending on the PVDFconcentration, the solvent, the concentration of additive b and thetemperature of the solution. Preferably, however, 2-20% of additive,based on the PVDF content, is used.

The addition of additive b influences the pore size to a lesser extent,it is, however, predominantly added because of its viscosity-increasingproperties. The viscosity is important for the spreading on thenon-woven support material. If the viscosity is too low, the solutionpenetrates deeply into the support layer after having been applied andcauses surfaces defects because of insufficient cover. Too highviscosity, on the other hand, causes minimal penetration and thusinsufficient adhesion to the support layer. The swelling effect of theadditive on the solution causes a larger porosity in the final membranebut also a decrease of its mechanical strength. Preferably 5-25% ofadditive, based on the PVDF-content, is used.

The spreading parameters may be varied and especially the precipitationtemperature influences the pore size.

Example of the preparation of MF support membranes

A solution is prepared comprising 18.5% polyvinylidene fluoride (Dyflor2000® from Dynamit Nobel), 1,5% chlorotrifluoroethylene/vinylidenefluoride copolymer ("Kel-F" 800 Resin from 3M Company) and 80%N-methylpyrrolidone. Then 5% polyvinylpyrrolidone (Kollidon® 90 fromBASF), based on the total amount of the solution, is added. The solutionis spread 0.15 mm thick on a non-woven polypropylene material beingconveyed on an endless steel band at a speed of 6.5 m/min. After approx.5 sec the spread layer is precipitated by means of dipping into a waterbath of 25° C. to form a microfiltration membrane. The waterpermeability of the membrane was 862 l/m² /h, measured for pure water ina RO Division Lab. 20 module of De Danske Sukkerfabrikker (DDS). Afterdrying the bubble point was measured to be 250 mbar, using 2-propanol aswetting agent.

The invention is further explained in the following examples.

EXAMPLE 1

An aqueous solution is prepared comprising 0.5% by weight/volume ofhydroxyalkylcellulose as well as 1% by weight/volume of NaOH.

An asymmetric UF membrane made of PVDF and with a cutoff value of 30,000(DDS RO membrane type FS50PP) is streched on a steel frame and dippedinto the above solution for 2 min. The membrane is then removed from thesolution and kept in a vertical position for 1 min so that excess liquidmay drain off, whereupon the membrane is placed in a ventilated oven for15 min at 150° C.

The membrane is tested for the ability to be wetted by measuring thecontact angle with water, obtaining the following result:

    contact angle=25.5°

Measuring the contact angle of an untreated, dry membrane gives thefollowing result:

    contact angle=68.9°.

EXAMPLES 2.4

Different aqueous solutions are prepared, their compositions beingdescribed in TABLE 1 below. The membrane type FS50PP is treated asdescribed in Example 1 and the following results are obtained:

                  TABLE 1                                                         ______________________________________                                        Example                                                                       No.     Composition        Contact Angle                                      ______________________________________                                        2       0.5% hydroxypropylcellulose                                                                      43°                                                 1% NaOH                                                               3       0.5% hydroxyethylcellulose                                                                        0°                                                 1% NaOH                                                                       0.5% 4-aminophenol                                                    4       0.5% hydroxypropylcellulose                                                                      56°                                                 1% NaOH                                                                       1% 1,3-dichloro-2-propanol                                            ______________________________________                                    

EXAMPLES 5-8

In stead of an asymmetric UF membrane an asymmetric MF membrane is usedin these examples, said MF membrane comprising a mixture of PVDF andCTFE/VF-copolymer but with a bubble point 500 mbar measured with wateras wetting agent.

The procedure is as described in Example 1, the dipping period, however,being 5 min and in Example 7 15 min.

The results are shown in TABLE 2.

                  TABLE 2                                                         ______________________________________                                        Example                  Contact  Bubble                                      No.    Composition       Angle    Point (H.sub.2 O)                           ______________________________________                                        5      0.4% hydroxyethylcellulose                                                                      29°                                                                             370                                                1% NaOH                                                                       0.5% SDS                                                               6      0.2% hydroxyethylcellulose                                                                      48°                                                                             380                                                1% NaOH                                                                       1% 2-aminophenol                                                              0.5% SDS                                                               7      as Example 6      10°                                                                             380                                         8      untreated, reference                                                                            76°                                                                             500                                         ______________________________________                                    

EXAMPLES 9-12

These examples illustrate changes in the properties of the UF membraneswhen treated according to the invention with regard to cutoff value andinfluence of fouling materials.

For the experiments an asymmetric PVDF ultrafiltration membrane with acutoff value of 20,000 (DDS RO type FS61PP) is used. Solutions areprepared as described in TABLE 3 below and the treatment is performed asdescribed in Example 1, the oven treatment may, however, vary as shown.The membranes are tested in the laboratory module (lab. unit 20) of DDSRO Division by measuring the flux and permeability data of differenttest solutions. The results are shown in TABLES 4 and 5.

                  TABLE 3                                                         ______________________________________                                        Example                   Oven treatment                                      No.    Composition        Temperature/Time                                    ______________________________________                                        9      0.6% hydroxypropylcellulose                                                                      150° C./5 min                                       2% NaOH                                                                10     as Example 9       150° C./30 min                               11     1% hydroxyethylcellulose                                                                         150° C./15 min                                      2% NaOH                                                                       0.5% 1,3-dichloro-2-propanol                                                  0.5% SDS                                                               12     0.2% hydroxypropylcellulose                                                                      150° C./15 min                                      1% NaOH                                                                       0.5% 1,3-dichloro-2-propanol                                                  0.5% SDS                                                               ______________________________________                                    

                  TABLE 4                                                         ______________________________________                                        Press/      Water   10% Sugar Solution                                        Ex.    Temp     Flux    Press/Temp                                                                             flux  % permea-                              No.    bar/°C.                                                                         l/m.sup.2 /h                                                                          bar/°C.                                                                         (lm.sup.2 /h)                                                                       bility                                 ______________________________________                                        9      5/20     95      5/20     53    88                                     10     5/20     26      5/20     19    82                                     11     10/20    43      10/20    27    69                                     12     5/20     41      5/20     22    83                                     FS61PP-                                                                              2/20     169     5/20     151   95                                     untreated                                                                     ______________________________________                                    

                  TABLE 5                                                         ______________________________________                                        Measurement of the water flux before and after the                            membrane has been exposed to a 5% whey powder solution                        having recirculated in the sysgtem for 1 h at a pressure of 5 bar             and a temperature of 20° C.                                            Example   Water Flux   Water Flux Flux                                        No.       before l/m.sup.2 /h                                                                        after l/m.sup.2 /h                                                                       Decrease                                    ______________________________________                                        9         95           86         9%                                          10        26           25         4%                                          11        43           39         9%                                          12        41           41         0%                                          FS61PP    160          72         55%                                         (reference)                                                                   ______________________________________                                    

EXAMPLES 13-24 and REFERENCE EXAMPLES 1-1, 2-1 and 2-2.

These examples illustrate the changes in flux properties of themicrofiltration membranes when treated according to the invention andsubsequently tested on a "known" protein solution (skimmed milk).

The composition of the treatment solutions, the dipping times and theoven treatment are shown in TABLE 6. Two types of microfiltrationmembranes are used. Type 1 has an isopropanol bubble point at 250 mbarand Type 2 has an isopropanol bubble point at 150 mbar.

                                      TABLE 6                                     __________________________________________________________________________                               Dipping                                                                            Oven                                          Ex.                                                                              Membrane                                                                            Composition of treat-                                                                           time treatment                                     No.                                                                              type  ment solution     min  °C./min                                __________________________________________________________________________    13 1     0.4% hydroxypropylcellulose                                                                     2    150/15                                                 1% NaOH                                                                       2% 1,3-dichloro-2-propanol                                           14 2     as Example 13 but 0.5% SDS                                                                      2    150/15                                                 added                                                                15 1     0.4% hydroxyethylcellulose                                                                      2    150/15                                                 0.4% polyacrylic acid (MW 5000)                                               0.5% conc. sulphuric acid                                            16 1     0.2% hydroxyethylcellulose                                                                      2    150/15                                                 2% 2,6-diaminopyridine                                                        2% 1,3-dichloro-2-propanol                                                    1% NaOH                                                              17 2     0.4% hydroxyethylcellulose                                                                      2    150/15                                                 2% triallyl-1,3,5-triazine-                                                   2,4,6-trione                                                                  0.5% conc. sulphuric acid                                                     0.2% potassium peroxosulphate                                                 0.5% SDS                                                             18 1     1% hydroxyethylcellulose                                                                        2    150/15                                                 2% 2-aminophenol                                                              2% NaOH                                                              19 1     0.5% hydroxypropylcellulose                                                                     15   150/15                                                 1% NaOH                                                              20 1     as Example 19, but                                                                              15   150/15                                                 0.5% tetraethylene pentamine added                                   21 1     0.5% hydroxypropylcellulose                                                                     1    150/15                                                 2% sulfanilic acid                                                            1% NaOH                                                              22 2     0.5% hydroxyethylcellulose                                                                      1    150/15                                                 0.5% triallyl-1,3,5-triazine-                                                 2,4,6-trione                                                                  0.5% conc. sulfuric acid                                             23 1     0.4% hydroxypropylcellulose                                                                     15   150/15                                                 1% NaOH                                                                       0.5% 1,3-dichloro-2-propanol                                         24 1     5% sulfanilic acid                                                                              2    150/15                                                 2% NaOH                                                              1-1                                                                              1     untreated                                                            2-1                                                                              2     untreated                                                            2-2                                                                              2     untreated                                                            __________________________________________________________________________

                  TABLE 7                                                         ______________________________________                                        Test data: 20 cm lab. module, flux = 8 l/min, temperature                     20° C.                                                                 Ex.                 Pressure Flux  Dry matter                                 No.  Test Solution  bar      l/m.sup.2 /h                                                                        permeability %                             ______________________________________                                        13   skimmed milk   2        296   97                                         14   skimmed milk   2        621   100                                        14   5% whey powder sol.                                                                          5        100   98.5                                       15   skimmed milk   2        224   95                                         16   skimmed milk   2        316   99                                         17   skimmed milk   2        724   100                                        17   5% whey powder sol.                                                                          5        110   97                                         18   skimmed milk   2        272   97                                         19   skimmed milk   2        250   99                                         20   skimmed milk   2        352   100                                        21   skimmed milk   2        310   100                                        22   5% whey powder sol.                                                                          5        172   100                                        23   skimmed milk   2        259   98                                         24   skimmed milk   2        200   93                                         1-1  skimmed milk   2        75    82                                         2-1  skimmed milk   2        414   99                                         2-2  5% whey powder sol.                                                                          5        86    85                                         ______________________________________                                    

As shown in TABLE 7 the microfiltration membranes treated according tothe invention possess considerably improved properties compared to theuntreated membranes with regard to flux and dry matter permeability, andadsorption phenomena are thus less important. These conditions are ofgreat importance for an industrial application of the microfiltrations.

EXAMPLE 25

An asymmetric UF membrane made of PVDF with a cutoff value of approx.30,000 MW is subjected to the following surface treatment.

An aqueous solution comprising 0.2% hydroxypropylcellulose (MW 100.000),4% sulfanilic acid and 4% NaOH is poured on the surface. After approx. 1min the excess liquid is removed by means of a squeeze roller and themembrane is transferred to an oven at 120° C. for 5 min.

A UF test in the lab. unit 20 of DDS RO Division revealed the followingresults by measuring on skimmed milk as well as on whey powder solution(and on the untreated basic membrane as reference):

lab. 20, flux 8 l/min, temp: 20° C., pressure: 4 bar, flux measuredafter 1 h recirculation

    ______________________________________                                                   Membrane acc. to                                                                         Reference mem-                                                     Example 25 brane, untreated                                        ______________________________________                                        skimmed milk 53 l/m.sup.2 /h                                                                            42 l/m.sup.2 /h                                     5% whey powder                                                                             70 l/m.sup.2 /h                                                                            56 l/m.sup.2 /h                                     solution                                                                      ______________________________________                                    

EXAMPLES 26-29

These examples illustrate the changes in the properties of a UF membranewhen treated according to the invention with respect to permeabilityproperties and achieved flux value under filtration of skimmed milk. Thelatter shows improved values compared to existing UF membranes withcomparable permeability properties.

The support membranes used in Examples 26-29 are either polyvinylidenefluoride (PVDF)-based or polyethersulfon (PESF)-based. The compositionof the solutions used as well as the oven treatment time and the oventemperature are shown in TABLE 8. Using the same method as described inExample 25 hydrophilic, surface-treated UF membranes are prepared. Themeasuring results and the test solutions are described in TABLE 9. Dataof known, untreated UF membranes (DDS type GR⁶¹ PP, GR⁸¹ PP, and GR⁹⁰PP) are also shown in TABLE 9.

                  TABLE 8                                                         ______________________________________                                                                           oven                                       Example                                                                              support    treatment solution                                                                             treatment                                  No.    membrane   composition      °C./min                             ______________________________________                                        26     PESF       1% hydroxyethyl- 120/9                                                        cellulose                                                                     2.5% inositol                                                                 2.5% conc. sulphuric                                                          acid                                                        27     PVDF       0.25% hydroxypropyl-                                                                           120/9                                                        cellulose                                                                     MW 1,000,000                                                                  1.25% inositol                                                                1.25% conc. sulphuric                                                         acid                                                        28     PESF       0.5% hydroxypropyl-                                                                            120/9                                                        cellulose                                                                     MW 1,000,000                                                                  5% gluconic acid                                                              3.75% conc. sulphuric                                                         acid                                                        29     PESF       0.25% polyvinylalcohol                                                                         120/9                                                        MW 125,000                                                                    0.5% penterythritol                                                           0.5% 4,5-dihydroxy-1,3-                                                       benzenedisulfonic                                                             acid (Tiron ®)                                                            0.5% TWEEN ® 20                                                           3.75% conc. sulphuric                                                         acid                                                        ______________________________________                                         TWEEN ® 20 = polyoxyethylenesorbitane ester                          

                                      TABLE 9                                     __________________________________________________________________________    Measuring results:                                                            DDS lab. 20 module, flux 8 l/min. temp. 20° C.                                                           flux                                        % permeability.sup.1)             (l/m.sup.2 /h)                              Ex.  sucrose                                                                           dextran (0.5%)                                                                             PFP.sup.2) (5%)                                                                     vitamin B.sub.12                                                                    skimmed milk                                No.  (10%)                                                                             MW 10,000                                                                           MW 4-6,000                                                                           MW 23,000                                                                           (50 mg/l)                                                                           2 bar                                                                             4 bar                                   __________________________________________________________________________    26   98  23           28          34  41                                      27   91  15           28          38  41                                      GR61PP                                                                             96  15           22          23  37                                      28   91        44     7           6.0 9.5                                     GR81PP                                                                             91        40     9           4.3 6.9                                     29   81        6            27    13.8.sup.3)                                                                       25.4                                    GR90PP                                                                             79        8            23    2.6.sup.3)                                                                        5.2                                     __________________________________________________________________________     .sup.1) measured at a pressure of 5 bar except for dextran, MW 10,000, th     latter was measured at 2 bar                                                  .sup.2) PVP -- polyvinylpyrrolidone                                           .sup.3) measured at 5 bar                                                     .sup.4) measured at 10 bar                                               

REFERENCE EXAMPLE 3 Example of the preparation of MF support membranemade of CTFE/VE-copolymer.

A homogeneous N-methyl-2-pyrrolidone solution comprising 25%chlorotrifluoroethylene (CTFE)/vinylidene fluoride (VF)-copolymer (3MCompany-Kel F 800 resin) is prepared. 5% polyvinylpyrrolidone (Kollidon®90 from BASF) is added and completely dissolved in said copolymer toform a homogeneous casting solution.

The solution is spread by means of a knife on a non-woven polypropylenematerial (100 g/m² -thickness approx. 200 micron) to form a 150 micronthick layer conveyed on an endless steel and with a speed of 6.7 m/min.After approx. 5 s the material is dipped into water of 25° C. Thus thepolymer is precipitated and forms a microfiltration membrane. Themembrane has a bubble point in 2-propanol of 150 mbar.

EXAMPLES 30 AND 31 Microfiltration Membrane Made of Fluoro-copolymer asSupport Membrane

A microfiltration membrane is prepared from a CTFE/VF-copolymeraccording to the reference example 3 and with a bubble point in2-propanol of 150 mbar and is subjected to the same treatment asdescribed in Example 1, using the following aqueous solutions:

    ______________________________________                                        Example                  oven treatment                                       No.       composition    temp./time                                           ______________________________________                                        30        5% sulfanilic acid                                                                           125° C./15 min                                          5% NaOH                                                             31        0.5% hydroxypropyl-                                                                          125° C./15 min                                          cellulose MW 100,000                                                          5% sulfanilic acid                                                            5% NaOH                                                             ______________________________________                                         Test data: 20 cm lab. module, flux 8 l/min, temp. 20° C.              Ex.    test      pressure   flux  dry matter                                  No.    solution  bar        l/m.sup.2 /h                                                                        permeability                                ______________________________________                                        30     5% whey   2          164   100%                                               powder sol.                                                                   5% whey   4          417   100%                                               powder sol.                                                            31     5% whey   2          114   100%                                               powder sol.                                                                   5% whey   4          266   100%                                               powder sol.                                                            ref*   5% whey   2          31     95%                                               powder sol.                                                                   5% whey   4          153   100%                                               powder sol.                                                            ______________________________________                                         *untreated CTFE/VF support membrane as used in Examples 30 and 31.       

I claim:
 1. A permeable, porous polymeric ultrafiltration ormicrofiltration membrane with hydrophilic character comprising: apermeable porous polymeric membrane; and a hydrophilic surface layerhaving hydrophilic character deposited on the porous polymeric membrane;obtained by the treatment of the porous polymeric membrane surface witha solution comprising one or more hydrophilic mono- or polymericcompounds selected from the group consisting of soluble cellulosederivatives, polyvinyl alcohols, and low molecular weightpolyfunctional, NH or OH-- containing compounds, followed by renderingthe layer deposited during the treatment insoluble on the porouspolymeric membrane surface by means of a base catalysed reaction atelevated temperatures in order to chemically bind the hydrophilicsurface layer to the porous polymeric membrane; wherein the hydrophilicsurface layer predominantly determines the filtration properties. 2.Membrane according to claim 1 wherein the cellulose derivative iscompletely or partially substituted by one or more low molecular weight,polyfunctional compounds selected from the group consisting ofdiaminopyridines and primary and secondary amines, of the generalformula ##STR3## where R is --NMR², --OH, --SO₃ H or --COOH, and R¹ andR² are independently either H or an alkyl group.
 3. Membrane accordingto claim 1 wherein the low molecular weight, polyfunctional compoundsare selected from the group consisting of diaminopyridines and primaryand secondary amines, of the general formula ##STR4## where R is --NHR²,--OH, --SO₃ H or --COOH, and R¹ and R² are independently either H or analkyl group.
 4. The membrane according to claim 3 wherein the treatmentsolution comprises one or more soluble cellulose derivatives and one ormore low molecular weight polyfunctional NH- or OH containing compounds.5. The membrane according to claim 4 wherein the treatment solution iswater based.
 6. The membrane according to claim 5 wherein the cellulosicderivative is water soluble and selected from the group consisting ofhydroxyethyl cellulose, hydroxymethyl cellulose, methyl cellulose, ethylcellulose, carboxymethyl cellulose and methyl hydroxyethyl cellulose. 7.A membrane according to claim 6 wherein the low molecular weight,polyfunctional NH- or OH-containing compound is selected from the groupconsisting of diaminopyridines, tetraethylene pentamine, aminophenolsand sulfanilic acid.
 8. A membrane according to claim 7 wherein thesoluble cellulose derivative is selected from the group consisting ofhydroxyethyl cellulose, and hydroxypropyl cellulose; and the lowmolecular weight polyfunctional NH- or OH containing compound isselected from the group consisting of sulfanilic acid,2,6-diaminopyridine, 2-aminophenyl and 4-aminophenol; and the base issodium hydroxide.
 9. A membrane according to claim 1, wherein thetreatment of the membrane surface is performed in the presence of atleast one cross-linking agent, surfactant or initiator.
 10. A membraneaccording to claim 9 wherein the treatment of the porous polymericmembrane surface is performed in the presence of a crosslinking agent.11. A membrane according to claim 10 wherein the treatment solution iswater based.
 12. A membrane according to claim 11 wherein thehydrophilic mono- or polymeric compound is a water soluble cellulosederivative selected from the group consisting of hydroxyethyl cellulose,hydroxymethyl cellulose, methyl cellulose, ethyl cellulose,carboxymethyl cellulose, and methyl hydroxyethylcellulose.